This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / SWAT Kats: The Radical Squadron
The destruction of Cybertron, the heroes' Robot Buddy in "The Deadly Pyramid". He was supposed to get better, but they never got around to it.
Professor Hackle did mention it would take years to fix him.
Also, the fact that this show may never get a revival. An interview with Lance Falk (an artist and writer for the series) confirmed that the chances of it are "zero to less than zero". DVDs are now available, albeit not remastered. Then again, Japan has made anime of The Powerpuff Girls and Lilo & Stitch, so someday there might be an anime for this too.
Happy Tear Jerker: There's a kickstarter for a revival and it's doing great with most of the surviving cast members saying they'd be thrilled to reprise their roles.